The process whereby bulk X—Ba—Cu—O material (where X comprises at least one rare-earth element such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y) high-temperature superconductors (HTS) are manufactured has been the subject of considerable scientific development over the last ten years. For the avoidance of doubt, in this document the term rare-earth element includes Y as well as the lanthanides.
Large-grain bulk X—Ba—Cu—O materials have significant potential for generating large magnetic fields, in excess of those achievable with conventional permanent magnets, for a variety of engineering applications such as magnetic bearings, MRI (magnetic-resonance imaging) and flywheel energy-storage applications.
One known technique for producing bulk superconductors is top-seeded melt growth (TSMG), in which a seed crystal is used to control the heterogeneous nucleation of a large superconductor grain. The TSMG technique is generally used to fabricate large single-grain superconductors and also very large superconductor tiles, using multiple seeds.
Since X—Ba—Cu—O materials are iso-structural and their melting point increases as the ionic radius of the rare-earth element(s) X decreases, higher-melting-point materials or systems can be used as seed crystals for lower-melting-point materials or systems. The melting point or peritectic decomposition temperature (Tp) is about 1070° C. (±10° C.) for La, Nd, and Sm systems, decreasing to about 980° C. for the Yb—Ba—Cu—O system.
In order to fabricate large, single-grain Y—Ba—Cu—O superconductors, whose melting point is about 1010° C., small crystals of the Nd—Ba—Cu—O or Sm—Ba—Cu—O systems are generally used as seed crystals in the TSMG method. Large tiles of Y—Ba—Cu—O superconductors have been processed successfully for various applications using this method.
LRE (light-rare-earth) systems can be denoted (LRE)-Ba—Cu—O and LRE elements include, for example, La, Nd, Sm, Eu, and Gd. Mixed-rare-earth (MRE) systems include more than one rare-earth element, for example, (Y, Nd)—Ba—Cu—O or (Nd, Sm, Gd)—Ba—Cu—O.
The superconducting properties, such as transition temperature (Tc), critical current density (Jc) and irreversibility field (Hirr), of LRE systems or MRE systems are generally superior to that of the Y—Ba—Cu—O system. For MRE systems, enhanced levels of irreversibility field and critical current density may be achieved if one or more of the rare earth elements incorporated is or are light-rare-earth elements.
Although the properties of (LRE)-Ba—Cu—O materials are particularly favourable for potential superconducting applications, (LRE)-Ba—Cu—O samples have not been routinely fabricated in the form of single grains using the TSMG process. This is due primarily to the unavailability of a suitable seed crystal, i.e. a seed crystal having the same crystal structure as the desired superconductor and a higher melting point or Tp. If a seed crystal does not have a higher melting point than the desired product superconductor it melts during TSMG fabrication and, therefore, does not seed the growth of the superconductor.
One seed crystal that has been used to fabricate high-melting-point superconductor systems is MgO. This material has a 22% lattice mismatch with the X—Ba—Cu—O system, which is a disadvantageously large mismatch, and also suffers from wetting problems. Therefore, it has low success in producing a desired bulk LRE superconductor and it is not a viable commercial seed crystal.
To overcome the seed crystal problem, many researchers have used a technique called “hot seeding”. In this method an X—Ba—Cu—O sample is first melted at well above its Tp and then cooled to just above the XBa2Cu3Od solidification temperature before a small X—Ba—Cu—O seed crystal of the same material is added to the sample surface.
This method is not very practical for processing on an industrial scale, however, because it needs a specially-designed furnace to place a seed crystal at the required position on the melt at elevated temperature while controlling the processing atmosphere. Controlling the processing atmosphere is very important and is critical in inhibiting the formation of a solid solution, in which the LRE element substitutes onto the Ba site, or vice versa. In addition, a small perturbation in oxygen content may affect the seeding process and also depress the material's Tc, thereby yielding a fully-processed sample with poor superconducting properties.